Frozen, lightweight curable sealant

ABSTRACT

A uniformly premixed sealant is extruded into a packaging form and allowed to pressure equilibrate prior to freezing. Upon warming the sealant begins to cure. The storage of the preformed sealant formulation under cold conditions arrests the activity of cure catalysts and accelerators mixed therethrough. The sealant is readily extruded into a preform shape containing a lightweight filler. A release film is provided to avoid contact with the sealant preformed in a shape during application to a substrate.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/369,407 filed Feb. 20, 2003, which claims priority ofProvisional Patent Application No. 60/360,555 filed Feb. 26, 2002. Thisapplication also claims priority of Provisional Patent Application No.60/510,975 filed Oct. 14, 2003, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to multi-component, ambient curing sealantand adhesive compositions suitable for use in sealing and/or bondingcomponents and, more particularly, to methods of sealing aircraftsubassemblies using preformed, curable sealant.

BACKGROUND OF THE INVENTION

Thiol-terminated sulfur-containing polymers have a long history of usein aerospace sealants because of their fuel resistant nature uponcross-linking. Among the commercially available polymeric compoundshaving sufficient sulfur content to exhibit this desirable property arethe polysulfide polymers described, e.g., in U.S. Pat. No. 2,466,963 andsold under the trade name Thioplast® polysulfide (Akcros Chemicals,Germany); U.S. Pat. No. 4,366,307 sold in complete sealant formulationsby PRC-DeSoto International, Inc. of Glendale, Calif.; andPCT/US01/07736, PCT/US01/07737, and PCT/US01/07738. In addition to fuelresistance, polymers useful in this context must also have the desirableproperties of low temperature flexibility, liquidity at roomtemperature, high temperature resistance, a reasonable cost ofmanufacture, and not be so malodorous as to prevent commercialacceptance of compositions that contain the subject polymers.

An additional desirable combination of properties for aerospace sealantswhich is much more difficult to obtain is the combination of longapplication time (i.e., the time during which the sealant remainsusable) and short curing time (the time required to reach apredetermined strength). Singh et al., U.S. Pat. No. 4,366,307, disclosesuch materials. Singh et al. teach the acid-catalyzed condensation ofhydroxyl-functional thioethers. The hydroxyl groups are in thebeta-position with respect to a sulfur atom for increased condensationreactivity. The Singh et al. patent also teaches the use ofhydroxyl-functional thioethers with pendent methyl groups to affordpolymers having good flexibility and liquidity. However, the disclosedcondensation reaction has a maximum yield of about 75% of the desiredcondensation product. Furthermore, the acid-catalyzed reaction ofβ-hydroxysulfide monomers yields significant quantities (typically notless than about 25%) of an aqueous solution of thermally stable andhighly malodorous cyclic byproducts, such as 1-thia-4-oxa-cyclohexane.As a result, the commercial viability of the disclosed polymers islimited.

Another desirable feature in polymers suitable for use in aerospacesealants is high temperature resistance. Inclusion of covalently bondedsulfur atoms in organic polymers has been shown to enhance hightemperature performance. However, in the polysulfide polyformal polymersdisclosed in U.S. Pat. No. 2,466,963, the multiple —S—S— linkages resultin compromised thermal resistance. In the polymers of Singh et al., U.S.Pat. No. 4,366,307, enhanced thermal stability is achieved throughreplacement of polysulfide linkages with polythioether (—S—) linkages.In practice, however, the disclosed materials also have compromisedthermal resistance due to traces of the residual acid condensationcatalyst.

Yet another desirable feature of materials useful as aircraft sealantsis the ability of the polymeric system to cure or cross-link underambient conditions. For the purposes of this application, the term“ambient conditions” refers to temperatures and humidity levelstypically encountered in an aircraft manufacturing environment. Numerouspotentially useful cross-linking reactions occur at ambient conditions.

To achieve the required blend of application and performance properties,current commercial products are usually multi-component sealants. Widelyaccepted aerospace sealants consist of a first component containing atleast one ungelled thiol terminated sulfur containing polymer and asecond component containing either an oxidizing compound, e.g.,manganese dioxide or any of a variety of Cr(VI) compounds, or a thiolreactive material, e.g. a polyepoxide, polyene or polyisocyanate.Optionally, both the first and second components also contain one ormore formulating ingredients chosen from the list of fillers, pigments,plasticizers, stabilizers, catalysts, activators, surface-activecompounds, solvents, and adhesion promoters. The types and quantities ofthese later ingredients are chosen and adjusted such that specificproperties are achieved. Any of a number of other reactive groups, e.g.,hydroxyl-, amine-, acryloxy-, siloxy- and maleimide, may be introducedonto the sulfur containing polymer backbone. By proper choice of ambientcuring chemistry, these alternative reactive groups may be equallysubstituted for the aforementioned thiol functionality.

Polysulfide sealants may also be supplied in two-part injection kits,such as those manufactured by Techcon Industries and sold under thetrade name Techkit, or in single component cartridges as premixed andfrozen sealant (PMF). PMF sealants are highly desired by end users asthey eliminate the tedious and costly process of on-site mixing as wellas providing the end user with the convenience of a one-componentmaterial that has the fast cure rate typical of a two-component sealant.

Currently, the construction of both small- and large-scale aircraftsub-assemblies is entirely a manual operation. The quality and integrityof the sealed joint or seam is totally reliant on the skill and abilityof the sealing operator to: correctly and thoroughly measure and mix thedifferent sealant components; correctly apply the requisite amount ofsealant for each bond-line requiring sealing; and reproducibly, overtime, repeat this process on every sub-assembly requiring sealing.

In addition, it is highly desirable for each and every sealing operatorto seal each and every sub-assembly in exactly the same manner. Inpractice, airframe manufacturers have addressed this challenge throughcomprehensive training programs, detailed sealing procedures andnumerous in-process inspections. Despite these efforts, defects arecommon and resealing requires removing the part from the assembly-lineprocess, an action that substantially diminishes manufacturingefficiency.

Aircraft manufacturers, in an effort to increase the fuel economy,attempt to reduce the weight of every aircraft component. Sealantmanufacturers have responded to this demand for lower density sealantsby incorporating lightweight fillers. The lightweight fillers suitablefor use in aerospace applications may be classified as being organic,inorganic, or a hybrid composition. The particle morphology falls withintwo categories—hollow microspheres and amorphous filler particles.

The manufacture of premixed and frozen lightweight sealants is arelatively new, and not well understood, process. To those skilled inthe art, the longstanding process for preparing a premixed and frozenpolysulfide sealant involves the pumping of the base and curing agentcomponents, in the proper mix ratio, into a rotary mixing chamber. Themixed sealant is then extruded into cartridges that are flash frozen toa temperature below the liquid sealant's glass transition temperature.This flash freezing process step is accomplished by immediately placingthe sealant cartridge into a dry ice bath or into a liquid nitrogenfreezing chamber.

Unexpectedly, when premixed and frozen lightweight sealants areprepared, it is known that many of the physical properties of the thawedsealant are degraded. Thus, there exists a need for a process to producepremixed and frozen low density aerospace sealants having resistance tomercaptan-containing aviation fuels. An additional need exists for asealant that reduces labor and variability while affording ambient cure,shelf stability and light weight.

SUMMARY OF THE INVENTION

A premixed frozen sealant is prepared by mixing sealant componentsincluding a curable prepolymer and a curing agent to form asubstantially uniform premix. The premix is extruded into a packagingform and allowed a pressure equilibration rest prior to freezing. Thepremix is optionally extruded into a preform shape in contact with arelease film.

A preformed sealant package includes a curable uniformly premixedone-part sealant formulation which contains a lightweight filler and ispreformed to a shape adapted to secure to a substrate. A release film isin contact with the formulation. A commercial package includes suchpreformed sealant package along with instructions for the application ofthe preformed sealant to a substrate and allowing the sealantformulation to warm to a temperature-inducing cure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preformed sealant package according tothe present invention where the relative dimensions of release filmlayers are distorted for illustrative purposes;

FIG. 2 is a longitudinal cross-section of the inventive package depictedin FIG. 1; and

FIG. 3 is a transverse cross-section of the inventive package depictedin FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention satisfies a long-felt need in the aerospaceindustry by providing a uniformly premixed lightweight frozen sealantparticularly well suited to sulfur-containing sealants. The sealant isoptionally formed as a frozen preform that upon warming cures in thepreform shape as compressed against a substrate. This inventionaccommodates the many types and shapes of bond-lines as well as thediffering substrates now in use. Additionally, the present inventionsatisfies the various performance requirements now delivered byconventional aerospace sealants, while obviating the difficulties inobtaining a sealant with both a long work life and a short cure time.The present invention includes the method of mixing and degassing amulti-component sealant formulation to yield a material that will cureabsent further mixing or the addition of other components. The premixedsealant formulation is allowed a pressure equilibration rest periodprior to flash freezing. The formulation is optionally preformed into adesired shape prior to being frozen.

The present invention is described herein with respect tosulfur-containing lightweight sealants. Operative examplesillustratively include manganese dioxide and polysulfide, epoxy curedpolysulfide, epoxy cured polythioethers, isocyanate cured polysulfide,and isocyanate cured polythioethers and in particular isocyanate curedhydroxyl terminated polythioether. It is also appreciated that theinvention is also well suited for other polymeric adhesive or coatingsystems illustratively including polysiloxane, fluoropolymers, epoxies,polyurethanes, polyesters, polythioethers, acrylates, and phenolics.

A base sealant composition is formed from a polymerizablesulfur-containing prepolymer such as those disclosed in U.S. Pat. No.2,466,963, which is incorporated herein by reference. A plasticizer ispreferably added to improve working properties. Plasticizersconventional to the art are operative herein and illustratively includealiphatic oils, waxes, fatty acid salts, and resins derived fromalkylated phenols and esters. While the amount of plasticizer present isnot critical and indeed depends on variables illustratively includingthe molecular weight of the seal precursors and the physicalrequirements of the cured adhesive, in general the plasticizer ispresent from 0 up to about 40 parts by weight based upon sealantprecursor weight. It is further appreciated that a plurality ofcompatible plasticizers are operative herein.

To promote adhesion, resole phenolic resins may be used as described inU.S. Pat. No. 5,516,843. Typical phenolic resin additives are MethylonAP-108, Durez 16674, Bakelite BRL 3741, and Resinex 468. Bulk fillersare used to increase strength, impart needed rheological properties,reduce the cost or modify the density of the sealant. Tensile propertiescan be increased significantly, depending on the type of bulk filler,its particle size, and the type of cure. Typical bulk fillers includecalcium carbonate (wet or dry ground limestone, precipitated), carbonblacks (furnace, thermal), calcined clay, talc, silica and silicatefillers, and rutile titanium dioxide. Bulk fillers most typically occupyaround 40 weight percent of the sealant composition, but useful productsmay be obtained from materials containing from 2 to 80% by weight bulkfillers. Fillers also provide the multi-part uniform mixture withphysical properties helpful in adjusting the extrudability of theadhesive preform. Since the adhesive precursors are often low molecularweight liquids up to 10,000 Daltons, the fillers are optionallyincorporated into the two-part mixed composition to produce a thicker,easily extruded mass.

Pigments are also optionally included in the preformed sealant mixture.Pigments illustratively include titanium dioxide, zinc sulfide, carbonblack and various organic and inorganic substances known to the art.

In order to lower sealant density, a lightweight filler is added. Thelightweight fillers are typically hollow microspheres, amorphousmaterials or aerogels. The specific gravity of the microspheres rangesfrom about 0.1 to about 0.7 and are exemplified by polystyrene foam,microspheres of polyacrylates and polyolefins, and silica microsphereshaving particle sizes ranging from 5 to 100 microns and a specificgravity of 0.25 sold under the trademark ECCOSPHERES by W.R. Grace & Co.Other examples include alumina/silica microspheres having particle sizesin the range of 5 to 300 microns and a specific gravity of 0.7 soldunder the trademark FILLITE by Pluess-Stauffer International, aluminumsilicate microspheres having a specific gravity of from about 0.45 toabout 0.7 sold under the trademark Z-LIGHT, and calcium carbonate-coatedpolyvinylidene copolymer microspheres having a specific gravity of 0.13which are sold under the trademark DUALITE 6001AE by Pierce & StevensCorp.

The amorphous lightweight fillers typically have a specific gravityranging from about 1.0 to about 2.2, while an aerogel has a specificgravity of from 0.05 to 0.07. The amorphous lightweight fillers areexemplified by calcium silicates, fumed silica, precipitated silica, andpolyethylene. Examples include calcium silicate having a specificgravity of from 2.1 to 2.2 and a particle size of from 3 to 4 micronssold under the trademark HUBERSORB HS-600 by J.M. Huber Corp., and fumedsilica having a specific gravity of 1.7 to 1.8 with a particle size lessthan 1 micron sold under the trademark CAB-O-SIL TS-720 by Cabot Corp.Other examples include precipitated silica having a specific gravity offrom 2 to 2.1 sold under the trademark HI-SIL T-7000 by PPG Industries,and polyethylene having a specific gravity of from 1 to 1.1 and aparticle size of from 10 to 20 microns sold under the trademark SHAMROCKS-395 by Shamrock Technologies Inc. The amounts of the microspheric andamorphous lightweight fillers used in the lightweight sealant may befrom about 0.3 to about 10% and from about 4% to about 15% of the totalweight of the sealant, respectively.

Only recently, the specific gravity of aerospace sealants has beenreduced from the 1.6-1.8 range down to a minimum of approximately 1.0.Those skilled in the art have reached this specific gravity byincorporating fine hollow spheres and compensating for the loss inphysical strength by additions of more highly reinforcing fillers andpigments such as precipitated calcium carbonate or fumed silica.

Liquid polysulfide prepolymers operative in the present invention aremost typically cured by oxidizing the polymer's thiol (—SH) terminals todisulfide (—S—S—) bonds. The curing agents most commonly used are oxygendonating materials, preferably manganese dioxide, lead dioxide, calciumperoxide, zinc peroxide, cumene hydroperoxide, p-quinone dioxime andmixtures thereof. Most preferred is manganese dioxide. Lower valencemetallic oxide, other organic peroxides, metallic paint dryers andaldehydes may also function as curatives. The curing compound as theprincipal component of the second part of the composition of the presentinvention may be suspended in an inert organic plasticizer, such asHB-40, which is a hydrogenated perphenyl from Monsanto Corp., St. Louis,Mo., reaction modifiers and viscosity modifiers.

The first part or base compound and second part, curing agent, arecombined in a fixed and prescribed weight ratio in a range from about14:1 to about 5:1, preferably about 10:1 (first part to second part), toproduce the sealant. The sealant can be applied to the metal substrateby most conventional methods. These include spraying, brushing andextruding using two-part room temperature meter-mix equipment to combinethe components in the exact ratio. Hand mixing is also acceptable forsealant supplied in pints, quarts and gallon kits.

Freezing the sealant mixture immediately after combination of theprepolymer, first part and curing agent, second part precludes an enduser from having to assure homogenous mixing at the point of use.Additionally, premixed and frozen sealant is optionally preformed in avariety of shapes such as sheets and ribbons thereby allowing for a moreuniform and rapid application to a given substrate, as compared toextrusion from a dispensing tube.

According to the present invention, subsequent to mixing and packaging,a pressure equilibration rest period is added to the process prior toflash freezing. While the mechanism is unknown, it is believed thatsealants typically require at least 10 atmospheres of pressure to bepumped through the meter mixing equipment. The pressure equilibrationprocess step allows the “hollow” filler particles to expand to neartheir original volume before the flash freezing process begins. It isappreciated that equilibration to ambient atmosphere can also beexpedited by pulling a partial vacuum about the sealant.

The pressure equilibrium rest period typically lasts from 2 to 15minutes and depends on variables illustratively including filleridentity, filler quantity, sealant mixture viscosity, and meter mixingpressure. It is appreciated that longer rest periods are operativeherein so long as cure time is adjusted accordingly through modificationof curing agent identity and/or concentration in the sealant mixture.

Referring now to FIGS. 1-3, an inventive commercial package is showngenerally at 10. A premixed sealant formulation is formed by extrusioninto a preformed sealant 12. It is appreciated that in addition toextrusion, a preformed sealant is produced by molding, casting,die-cutting, and the like. In a preferred embodiment an inventivepreform is created that is independent of reinforcing metal or fabricwoven mesh reinforcement, as such a mesh tends to limit thedeformability to provide a press-fit workable seal. The sealant 12 isapplied onto a first release film 14. Typical preformed sealantthickness as measured perpendicular to an adherent substrate theretoranges from about 1 mm to 10 mm; however, it is appreciated thatdimensions beyond this range are also operative herein. Optionally, therelease film 14 is composed of a sealant contacting layer 20 laminatedto one or more structural layers 22. The first release film 14 is peeledfrom the preformed sealant 12 and the sealant 12 applied to a substrate.Preferably, an inventive preformed sealant 12 is sandwiched between thefirst release film 14 and a second like release film 16 to aid in sealalignment and to minimize handling thereof. It is appreciated that thesecond release film 16 is likewise optionally composed of a sealantcontacting layer 24 and one or more structural or external environmentbarrier layers 26 as detailed above with respect to the first releasefilm 14. Optionally, a bond 18 is provided by conventional means betweenrelease films to create a barrier encasing a preformed sealant. The bond18 being between portions of the first release film when only a singlefilm is present or between opposing edges of the first release film 14and the second film 16 in a dual release film embodiment depicted inFIGS. 1 and 3. The bond is formed through the application ofconventional adhesive, thermal bonding, sonic bonding or the like.

The preformed sealant 12 is a substantially uniform mixture of precisestoichiometry that cures upon thaw. Cure typically occurs after warmingthe preformed sealant to above about −60° C. to 4° C. Preferably, thepreformed sealant according to the present invention is packaged andstored with a cold pack having a melting temperature less than the curetemperature of the sealant in order to maintain the preformed sealant inan uncured state upon temporary interruption of cold storage conditions.

The nature of the curable sealant composition utilized herein is largelydictated by the resulting seal performance requirements. In a preferredaerospace embodiment, the sealant is a polythioether or polysulfidepolymer.

Various fillers are also optionally incorporated into the preformedsealant. Useful fillers illustratively include clay, talc, carbon black,silica and calcium carbonate.

At the point of use, an inventive commercial package is taken from coldstorage and used according to instructions that include the frozenpreformed sealant is separated from the release film and is applied to asubstrate with a consistency from tack-free to putty-like solid. Theworking time for the preformed sealant is utilized to shape andotherwise compression conform the extrudate to a desired form against asubstrate. An optional second release film is removed after finalforming. An additional substrate then optionally is brought into contactwith the preformed sealant to create a bond line. The sealant is thenthawed to a temperature sufficient to initiate the cure process. Thesubstrate relative position is maintained during cure to a completedseal. It is appreciated that the preformed sealant can be formed in avariety of shapes illustratively including ribbons, discs, sheets,cones, and more complex three-dimensional forms. Preferably, thepreformed sealant has a uniform dimension and weight per unit length.While ambient air atmosphere cure is common, it is also appreciated thatcuring ovens, vacuum ovens and other conventional cure conditions arealso operative herewith.

All patents and published applications disclosed herein are indicativeof the skill in the art. These patents and applications are hereinincorporated by reference to the same extent as if each werespecifically and individually incorporated by reference.

The foregoing description is illustrative of particular embodiments ofthe invention, but is not meant to be a limitation upon the practicethereof. The following claims, including all equivalents thereof, areintended to define the scope of the invention.

1. A preformed sealant package comprising: a curable uniformly premixedone-pan sealant formulation frozen at a temperature to preclude cureafter a pressure equalization rest period comprising an amorphouslightweight filler having a specific gravity of 1.0 to 2.2 or an aerogelhaving a specific gravity of 0.05 to 0.07 and preformed to a shapeadapted to secure to a substrate; and a first release film in contactwith said formulation, and wherein said sealant formulation comprises apolysulfide.
 2. The package of claim 1 further comprising a bond on saidfirst release film creating a barrier encasing said sealant formulation.3. The package of claim 1 further comprising a second release film incontact with said formulation.
 4. The package of claim 3 wherein saidfirst release film is in simultaneous contact with said formulation andsaid second release film.
 5. The package of claim 4 further comprising abond at a point of contact between said first release film and saidsecond release film.
 6. The package of claim 1 wherein said firstrelease film comprises a sealant contacting layer laminated to at leastone structural layer.
 7. The package of claim 1 wherein said shape isselected from the group consisting of: ribbon, disc, sheet, and cone. 8.The package of claim 1 wherein the sealant formulation further comprisesat least one additive selected from the group consisting of aplasticizer, a bulk filler, and a pigment.
 9. The package of claim 1farther comprising a cold pack.
 10. A commercial package comprising: apremixed, frozen one-part sealant formulation preformed into a shape ata temperature to preclude cure after a pressure equalization rest periodarid comprising an amorphous lightweight filler, an aerogel lightweightfiller, or a hollow microsphere lightweight filler having a specificgravity of between 0.1 and 0.7 along with instructions for the usethereof as a sealant, and wherein said sealant formulation comprises apolysulfide.
 11. The commercial package of claim 10 wherein the sealantformulation further comprises at least one additive selected from thegroup consisting of a plasticizer, a bulk filler, and a pigment.
 12. Thecommercial package of claim 10 further comprising a first release filmin contact with the sealant formulation.
 13. The commercial package ofclaim 12 further comprising a second release film in contact with thesealant formulation,
 14. The commercial package of claim 10 furthercomprising a cold pack.
 15. A preformed sealant package comprising: acurable uniformly premixed one-part sealant formulation frozen at atemperature to preclude cure after a pressure equalization rest periodcomprising an amorphous lightweight filler having a hollow microspherelightweight filler having a specific gravity of 0.1 to 0.7 and preformedto a shape adapted to secure to a substrate; and a first release film incontact with said formulation, And wherein said sealant formulacomprises a polysulfide.
 16. The package of claim 15 further comprisinga bond on said first release film creating a barrier encasing saidsealant formulation.
 17. The package of claim 15 further comprising asecond release film in contact with said formulation.
 18. The package ofclaim 17 wherein said first release film is in simultaneous contact withsaid formulation and said second release film.
 19. The package of claim15 wherein said first release film comprises a sealant contacting layerlaminated to at least one structural layer.
 20. The package of claim 15wherein said shape is selected from the group consisting of: ribbon,disc, sheet, and cone.
 21. The package of claim 15 wherein the sealantformulation further comprises at least one additive selected from thegroup consisting of a plasticizer, a bulk filler, and a pigment.
 22. Thepackage of claim 15 further comprising a cold pack.